Regional variation of convective structure at monsoon onset across South America inferred from TRMM observations

The goal of this paper is to study the changes in the vertical structure and horizontal organization of convective systems during the onset of the monsoon season in South America. In particular, we will contrast properties of convection for different regions where onset appears to be controlled by different large scale forcing mechanisms. The timing of monsoon onset across South America is complex, as the southward-moving intertropical convergence zone interacts with baroclinic systems that intrude equatorward from higher latitudes. This analysis will help to evaluate the roles of gradual thermodynamic priming of the atmosphere and rapid dynamical triggering (e.g. frontal systems) in establishing the monsoon in different regions of South America.

To examine regional changes in convective organization prior to and following onset, we analyzed the Tropical Rainfall Measuring Mission (TRMM) 3B42 0.25o 3-hourly merged precipitation maps for 1998-2007. Our purpose is to characterize the variability in the organization of precipitating cloud systems, such as mesoscale convective systems, large squall line systems, and frontal cloud systems at monsoon onset. Analysis of 3B42 imagery is used to produce regional statistics of system type and organization in each 5o x 5o land box. Changes in the organization and intensity of convective systems prior to and following onset are studied regionally via the metrics of vertical radar reflectivity structure, convective and stratiform rain rates, and lightning flash count using the TRMM Precipitation Feature database (Nesbitt et al. 2000) for 1998-2007. Time series, cumulative distribution functions, and composite analysis of these metrics prior to and following onset for different regions will be presented.